2nd member of homologous series having general formula CnH2n−2
CnH2n−2 is the general formula of alkyne.
Because in Alkynes there is a triple bond of C≡C. And as we all know Single Carbon (meth) has only one Carbon so it can't form a triple bond.
Therefore we can say that the first member will be having 2 carbon and hence it is Ethyne (ie. C2H2)
Whereas second member has 3 carbon atoms hence it is Propyne (ie.C3H4)